This invention is concerned with catalytic apparatus, particularly for the conversion of nitrogen dioxide to nitric oxide.
Instruments which measure the concentration of oxides of nitrogen by chemiluminescent techniques are well known. In general, such instruments detect the light produced by the reaction of ozone and nitric oxide which are mixed in the detector head, the light resulting from the reaction being sensed by a photomultiplier tube. Since the chemiluminescent reaction requires nitric oxide to work, if a concentration of nitrogen dioxide is to be measured, it must first be converted to nitric oxide. Hence, various types of converters have heretofore been proposed for use in instruments for determining oxides of nitrogen in ambient air or emission sources and also for use with calibration units having a nitrogen dioxide source.
Converters employing various types of catalytic materials are well known. However, the converters employed heretofore have suffered from one or more of the following disadvantages or deficiencies: high cost, restricted range in terms of the concentrations of NO.sub.2 which can be converted, short lifetime, high operating temperatures, production of interfering compounds, inefficiency, complexity, and inability to operate in the presence of other gases or under certain ambient conditions.